The present invention provides an assembly method which preferably is to be used in connection with the type of jig disclosed in application Ser. Nos. 216,702 and 262,894 (the '702 and '894 applications). That is, these applications respectively disclose assembly jigs for making wing panels and spars which hold together the pre-assembled parts of their respective wing components without using tack or other interference fasteners.
Briefly, these jigs permit the individual parts of their respective assemblies to expand or contract separately, in a defined, predetermined orientation, as they are subsequently assembled or combined with other parts by the use of bolts, rivets and other such interference fasteners. Without such individual freedom of part movement during assembly, the resultant or fastened-together assembly will have undesirable induced residual manufacturing stresses, as well as dimensional distortions. These residual stresses subtract from individual fastener joint strength as well as the overall strength of the assembly.
Using jigs that permit individual part movement as a way to avoid residual stresses during assembly is but a single aspect of a larger problem. It is also necessary to take into account the dimensional impact of all the part movements resulting from the installation of all fasteners required by the assembly.
During the course of assembly, the parts being assembled are constantly moving relative to each other, and the resulting assembly, or subassembly as the case may be, is also moving continually during the assembly sequence. This can be described as a "dynamic dimensional matrix system" that exists during the assembly operation. This latest invention disclosed here defines the necessary method for dynamically controlling assembly of such a matrix system that is applicable, but not necessarily limited to, the previously-referenced applications.
As was discussed in some depth in the '702 application, and to a certain extent in the '894 application, past methods of manufacturing wing components, where tack fasteners were used to hold parts together prior to final assembly, created induced stresses caused by part expansion that is restricted by the tack fasteners holding the parts together at common points. This results in compressive stresses in one part, tensile stresses in a second part, and shear stresses in the fasteners, as well as an overall subassembly expansion, and a torsional or bending stress in the completed subassembly. The nature of this problem is illustrated in FIGS. 14-16 herein, which are labeled "prior art."
Referring first to FIG. 14, there are many instances in an aircraft manufacturing operation where a first part a is initially joined or connected to a second part b by a plurality of fasteners c at opposite ends. Typically, parts a, b are elongated members that are finally assembled, subsequent to the end installations at c, by using numerous intervening fasteners distributed along their length.
As is indicated in FIG. 14, typically all the holes d, e for the intervening fasteners are drilled prior to their installation, and are drilled at the same time as the bores for the end fasteners c. At the time of such drilling, most of the bores d, e remain substantially in registration. However, installation of any interference fastener or a riveted fastener will coldwork and expand its respective hole or bore through each part (a and b) in the assembly which is joined together. Therefore, unless the cross-sectional area of each part (a and b) is identical, the amount of elongation of each will be different.
As illustrated by FIG. 14, part b has a smaller cross-sectional area than part a, and it will therefore elongate at a greater rate than a. Since the two parts are first fastened together at c, this causes an initial part expansion, and prevents further expansion created by subsequent installation of the intervening fasteners. This creates undesirable stored strain energy and residual stresses, and imposes shear stresses across the installed fasteners at the interface between the two parts.
By way of further explanation, and referring now to FIG. 15, as mentioned above, the fasteners c shown there are first installed at opposite ends of the parts a, b prior to installation of fasteners in the interval between. Depending on the nature and dimensions of the materials involved, it is not unusual for the end installations to initially cause unequal part expansions. In FIG. 15, for example, the greater expansion is schematically shown in part b, because of its smaller cross-section. This causes it to go into compression. At the same time, part a is placed in tension. The end result can be a slight camber in the parts a, b.
The predrilled bores d, e in the parts will experience axial shift because of the cambering effect described above, which takes them out of registration with each other. This not only adversely affects individual fastener installation of the intervening fasteners from the very beginning, but as such fasteners are subsequently installed, each individual fastener installation exacerbates the problem by making an individual contribution to part expansion. The result is additional induced stresses that create bending or warping of the type illustrated in FIG. 16.
The residual stresses created by this effect can be very significant and very undesirable. A person familiar with aircraft construction can appreciate the magnitude of the stresses between a spar chord and spar web, for example, where the ratios of the cross-sectional areas between the parts are over 1,000 to 1, and the length of the parts can be as much as 100 to 200 feet. Likewise, the ratios between wing stringers and a wing skin can exceed 5,000 to 1 over lengths of 100 to 200 feet. It is these kinds of problems which are addressed and solved by the present invention.
The invention provides dynamic dimensional control during the drilling and installation of interference fasteners. The invention does not inhibit individual part expansion due to sequential and progressive fastener installation. A dimensional matrix system is employed to continually detect and compensate for part movement to achieve preconceived dimensional values established for the completed assembly or subassembly. Detection and compensation for part movement on a continual basis either eliminates or drastically reduces induced stresses and residual strains. The invention will become better understood upon consideration of the following.